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<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <script>
        // 父类有加法方法
        class Father {
            constructor(x, y) {
                this.x = x;
                this.y = y;
            }
            sum() {
                console.log(this.x + this.y);
            }
        }
        // 子类继承父类加法方法 同时 扩展减法方法
        class Son extends Father {
            constructor(x, y) {
                // 利用super 调用父类的构造函数
                // super 必须在子类this之前调用
                super(x, y);
                this.x = x;
                this.y = y;

            }
            subtract() {
                console.log(this.x - this.y);

            }
        }
        var son = new Son(5, 3);
        son.subtract();
        son.sum();
    </script>
</body>

</html>